Symmetric keying and chain of trust

ABSTRACT

The present application is directed to symmetric keying and chain of trust. In one embodiment a prover may communicate with a verifier to perform device or data authentication. The verifier may issue a challenge to authenticate the prover, to which the prover may respond with key data and ticket data that may be forwarded to a certifier. If the ticket data is recognized, the certifier may transmit an authorization token to the verifier, which may then set a symmetric attestation key. The prover may utilize the symmetric attestation key to establish a chain of trust for use in responding to challenges to application or data authenticity. The symmetric attestation key may be used to generate a first level key that may be utilized to generate program keys. A challenge response may include at least program key data that may be authenticated by the verifier using the symmetric attestation key.

TECHNICAL FIELD

The present disclosure relates to device security, and more particularly, to allow for application and data security without substantial secure infrastructure existing in a prover device.

BACKGROUND

Preventing various devices from being compromised by malicious software or “malware” is becoming increasingly difficult as new strategies continue to emerge that circumvent existing security measures. For example, malware such as “rootkits” may seek to compromise security in a device by being loaded during device initialization. As a result, these rootkits may maintain a level of privilege in the device that exceeds even that of anti-virus software. Trusted Execution (TXT) is at least one security concept that may maintain security starting from initialization. In TXT a trusted platform module (TPM) may maintain “measurements” (e.g., results generated by cryptographic hash functions performed on at least part of a program's code) for known-good programs in a secure memory within the device. As programs are loaded into the device, they may be measured to determine if a known-good version of the program has been loaded. Any changes to the program would yield a different measurement, indicating that the program may have be malware or at least a version of the original program that has been altered by malware. In one embodiment, a “chain of trust” may also be instituted wherein each program measures a subsequently loaded program to ensure that all programs in the chain are known-good programs.

While effective to deter unauthorized access, TXT may require resources such as secure co-processors, secure memory, etc. to support attestation via asymmetric encryption keys. Such resources may be readily available in more robust platforms such as desktop computers, laptop computers and even mobile devices like tablet computers and smart phones. However, devices are now being developed that may not comprise these resources but may still be susceptible to being compromised by malware. For example, small format devices such as wearable devices, sensors and/or devices that may depend on energy harvesting (e.g., generating their own energy for available sources such as mechanical energy, solar energy, etc.) may comprise computing resources that may only be powerful enough to sustain the operation for which the device was designed. Without a low-level protection scheme enabling security measures such as integrity reporting (e.g., for verifying that a program is a known good version of the program) and sealing (e.g., for protecting software secrets such as encryption keys) that may be implemented via TPM, TXT, etc., it may be impossible to detect if a resource-constrained device has been compromised by malware, and consequently, to prevent the malware on the resource-constrained device from infecting other computing devices with which the resource-constrained device may interact.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of various embodiments of the claimed subject matter will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals designate like parts, and in which:

FIG. 1 illustrates an example system for symmetric keying and chain of trust in accordance with at least one embodiment of the present disclosure;

FIG. 2 illustrates example configurations for a verifier device and a prover device usable in accordance with at least one embodiment of the present disclosure;

FIG. 3 illustrates an example of symmetric key distribution in accordance with at least one embodiment of the present disclosure;

FIG. 4 illustrates an example of remote attestation in accordance with at least one embodiment of the present disclosure;

FIG. 5 illustrates example operations for symmetric keying and chain of trust from the perspective of a verifier in accordance with at least one embodiment of the present disclosure; and

FIG. 6 illustrates example operations for symmetric keying and chain of trust from the perspective of a prover in accordance with at least one embodiment of the present disclosure.

Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.

DETAILED DESCRIPTION

The present application is directed to symmetric keying and chain of trust. In one embodiment, a prover device may communicate with a verifier device to authenticate the identity of the prover device, applications executing on the prover device or data provided by the applications executing on the prover device. Interaction between the prover device and verifier device may occur in two phases. The verifier device may initially issue a challenge to authenticate the identity of the prover device, which may then result in symmetric attestation keys being stored on the prover and verifier devices. The challenge may involve the prover device providing at least key data and ticket data to the verifier device, which may then provide at least the ticket data to a certifier (e.g., at least one device corresponding to the manufacturer, distributor, seller, etc. of the prover device). If the ticket data is recognized, the certifier may transmit an authorization token to the verifier device allowing the symmetric attestation key to be set in the verifier using the key data and then confirmed to the prover. The symmetric attestation key may be employed by the prover device in establishing a chain of trust that may be used to respond when the verifier device challenges the authenticity of applications attempting to access the verifier device, the authenticity of data provided by applications in the prover device, etc. For example, the symmetric attestation key may be used to generate a first level key, which may in turn be used to generate program keys. A challenge response may include at least program key data that may be authenticated by the verifier device with the symmetric attestation key.

In one embodiment a device that may act as a verifier may comprise, for example, a communication module and a trusted execution environment (TEE). The TEE may include at least an attestation module to detect an attestation trigger, determine whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger and initiate at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.

Attestation may be triggered, for example, periodically or due to a request to access resources received from the prover device via the communication module. The communication module may be to interact with the prover device via at least one of a short-range wireless connection or a direct wired connection. The attestation module being to initiate the symmetric key distribution operations with the prover device may comprise the attestation module being to cause the communication module to transmit a device challenge to the prover device and receive a response to the device challenge from the prover device via the communication module, the response comprising at least key data and ticket data. The attestation module may then be further to cause the communication module to transmit at least the ticket data to at least one certifier device, receive a response from the at least one certifier device via the communication module and determine whether the response received from the certifier device comprises a authorization token. In one embodiment, the device may further comprise a firewall through which the communication module interacts with the at least one certifier device, the firewall being to at least prevent unintended interaction between the device and the at least one certifier device. The attestation module may be further to, if an authorization token is determined to have been received, set a symmetric key based at least on the key data and cause the communication module to transmit a confirmation that the symmetric key has been set to the prover device.

In the same or a different embodiment, the attestation module being to initiate the remote attestation operations with the prover device comprises the attestation module being to cause the communication module to transmit an application challenge to the prover device, receive a response to the application challenge from the prover device via the communication module, the response being generated based at least on counter data, code measurement data and program key data, authenticate the response and cause the communication module to transmit at least the results of the authentication to the prover device. The attestation module being to authenticate the response may comprise, for example, the attestation module being to authenticate that the response was generated based on the symmetric key using a message authentication code. An example method for remote attestation using symmetric keys consistent with the present disclosure may comprise detecting an attestation trigger, determining whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger and initiating at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.

FIG. 1 illustrates an example system for symmetric keying and chain of trust in accordance with at least one embodiment of the present disclosure. System 100 may comprise, for example, prover 102, verifier 104 and certifier 106. In general, verifier 104 may cooperate with certifier 106 to authenticate prover 102, applications running on prover 102, data provided by prover 102, etc. Prover 102 may be any device capable of interacting with verifier device 104 as illustrated in FIG. 1. Advantageously but not necessarily, the operations disclosed in regard to system 100 may allow prover 102 to be a resource limited device (e.g., limited processing power, energy storage, etc.) that would not otherwise be able to support existing attestation schemes such as a worn device (e.g., smart watch, eyewear, clothing, etc.), an implantable device (e.g., a device for monitoring a particular medical condition), a device that relies on energy harvesting (e.g., electromechanical generation, solar, etc.), a sensor device, an actuator device, etc. Verifier 104 may comprise a device with more substantial computing resources that may be capable of, for example, maintaining security via protection schemes based on TXT, TDM, etc. Examples of verifier 104 may comprise, but are not limited to, a mobile communication device such as a cellular handset or a smartphone based on the Android® OS from the Google Corporation, iOS® from the Apple Corporation, Windows® OS from the Microsoft Corporation, Mac OS from the Apple Corporation, Tizen OS from the Linux Foundation, Firefox OS from the Mozilla Project, Blackberry® OS from the Blackberry Corporation, Palm® OS from the Hewlett-Packard Corporation, Symbian® OS from the Symbian Foundation, etc., a mobile computing device such as a tablet computer like an iPad® from the Apple Corporation, Surface® from the Microsoft Corporation, Galaxy Tab® from the Samsung Corporation, Kindle Fire® from the Amazon Corporation, etc., an Ultrabook® including a low-power chipset manufactured by Intel Corporation, a netbook, a notebook, a laptop, a palmtop, etc., a typically stationary computing device such as a desktop computer, a server, a smart television, small form factor computing solutions (e.g., for space-limited applications, TV set-top boxes, etc.) like the Next Unit of Computing (NUC) platform from the Intel Corporation, etc. Certifier 106 may be an entity for certifying the identity of prover 102, and as such, may be, for example, the manufacturer of prover 102, a retailer or distributer of prover 102, etc. In one embodiment, certifier 106 may comprise at least one device accessible to verifier 104 via wired or wireless communication. For example, certifier 106 may include at least one server accessible via the Internet (e.g., organized in a cloud-computing configuration). For the sake of explanation herein, an example real-world application that may be used to envision system 100 may comprise at least one wearable sensor (e.g., prover 102) for monitoring a user's physiological performance, the wearable sensor being wirelessly coupled to a smart phone (e.g., verifier 104) in the user's pocket, the smart phone communicating with the manufacturer of the at least one wearable sensor (e.g., certifier 106) via a wireless Internet link.

A known solution for attestation may comprise embedding a certified public key into a trusted execution environment (TEE) in prover 102 that verifier 104 may use in authentication based on asymmetric keying (e.g., with assistance from certifier 106 that may interact directly with the TEE of prover 102). Given that performing elliptic curve cryptography (ECC) point multiplication can consume significant processing resources on low-end CPUs, and that existing protocols typically require more than one such operation, existing solutions may be too slow, costly, etc. for resource-constrained devices (e.g., it may take three point multiplications for establishing a session key with a Fully Hashed Menezes-Qu-Vanstone (FHMQV) protocol).

A corresponding symmetric solution (e.g., using Advanced Encryption Standard-Cypher-based Message Authentication Code (AES-CMAC)) may be many times faster, but requires a shared symmetric authentication key enabling certifier 106 to authenticate the TEE. While in both approaches certifier 106 may be trusted by all parties, allowing certifier 106 to participate in key negotiation means certifier 106 could later be used to impersonate prover 102 and spoof an attestation towards verifier 104. This vulnerability is addressed in embodiments consistent with the present disclosure wherein verifier 104 may isolate prover 102 during the key exchange to prevent interaction with certifier 106. As a result, verifier 104 and prover 102 can negotiate a key that can be authorized but not derived by certifier 106, and thus, may be secured against spoofing.

Consistent with the present disclosure, verifier 104 may initiate at least two categories of operations: symmetric key distribution 108 and remote attestation 112. For example, symmetric key distribution 108 involve interaction that results in the distribution of symmetric keys for use by prover 102 and verifier 104 during remote attestation 112. Initially, verifier 104 may issue device challenge 108A to prover 102. Device challenge 108A may be message that challenges prover 102 to “prove” its identity. Device challenge 108A may be triggered by a variety of events such as, for example, the activation of verifier 104, a periodic trigger in device 104, a request from prover 102 to access resources on verifier 104, the receipt of data provided by prover 102 at verifier 104, etc. Prover 102 may respond to verifier 104 as shown at 108B, the response including at least key data and ticket data. The key data may allow verifier 104 to confirm that response message 108B was received from prover 102 and may comprise at least a symmetric key generated by prover 102 that may be set in verifier 104 later during symmetric key distribution 108. The ticket data may be usable by an entity like certifier 106 to authenticate the identity of prover device 102. Verifier 104 may then forward at least the ticket data to certifier 106 as shown at 108C. In one embodiment, verifier 104 may be protected by firewall 110 that, for example, prevents intrusion from a local-area network (LAN), a wide-area network (WAN) like the Internet, a global-area network (GAN), etc. into the personal-area network (PAN), body-area network (BAN), etc. including at least prover 102 and verifier 104. Firewall 110 may physically or logically prevent unintended interaction between prover 102 and certifier 106 or other parties apart from verifier 104. Thus, verifier 104 may be assured that the deployed symmetric key is known only to prover 102 and verifier 104, and not to other parties outside of firewall 110. In the same or a different embodiment, the channel between verifier 104 and certifier 106 through firewall 110 may be a secure channel to ensure that the data being exchanged is not intercepted. As discussed above, the information forwarded to certifier 106 may be limited to the ticket data (e.g., only the data needed to verify the identity of prover 102) to ensure that certifier 106 does not have all of the data needed to impersonate prover 102.

Certifier 106 may utilize at least the ticket data to authenticate prover 102 (e.g., to ensure that another device is not impersonating prover 102). For example, the ticket data may comprise manufacturer data, model data, serial number, build date and/or dedicated verification data that may be used to verify that prover 102 is actually a product that was provided (e.g., manufactured, configured, distributed, sold, etc.) by certifier 106. If prover 102 is determined to be authentic, a response may be transmitted from certifier 106 to verifier 104 comprising at least authorization token as shown at 108D. Verifier 104 may use the authorization token in setting the symmetric key as shown at 108E. Setting the symmetric key may comprise, for example, designating the symmetric key for use in device/program/data authentication interactions with prover 102 and then storing the symmetric key in verifier 104 (e.g., in secure memory such as a TEE). Verifier 104 may then confirm that the symmetric key has been set to prover 102 as shown at 108F.

Remote attestation 112 may initiate in prover 102 with chain of trust derivation. For example, an initial framework key in the chain of trust may derived as shown at 112A based on symmetric key 114 that was provided to prover 102 as shown at 108E. Each subsequent key (e.g., program key 118) may be derived in part from at least the previous key (e.g., framework key 116) as shown at 112B. In this manner, various operational levels in prover 102 such as, for example, OS, applications, add-on modules, etc. may each have a key that may be authenticated using symmetric key 114. An example interaction is shown wherein application challenge 112C is transmitted by verifier 104 to prover 102. Application challenge 112C may include a message challenging an application to “prove” that the application resides on prover 102 and/or that data originated from prover 102. Similar to device challenge 108A, application challenge 112C may be triggered by an application on prover 102 requesting to access resources in verifier 104, data being provided from prover 102 to verifier 104, etc. Reply 112D may be transmitted in response to application challenge 112C, reply 112D comprising data based on at least program key 118 that may be authenticated in verifier 104 utilizing, for example, symmetric key 114.

Consistent with the present disclosure, chain of trust derivation does not require centralized recording of measurements that are trusted by all parties, and thus, devices do not require hardware-based protection to secure measurements or to report and sign them on behalf of others. Instead, software may be responsible for deriving keys correctly and Execution-Aware Memory Protection may be used to protect generated keys 116, 118, etc. from threats in the OS or other applications. Further to reduced hardware cost, chain of trust derivation may also mitigate the problem of performance bottlenecks during bootstrapping and trusted execution on resource constrained devices. The resulting attestation protocols require only a message authentication code (MAC) to prove the correct loading of a particular application, and may be easily and more efficiently integrated in secure channel protocols as compared to interoperating with central trusted subsystems (e.g., TPM). While symmetric chain of trust may be limited to provide only load-time measurement and attestation of individual (e.g., isolated) applications, it is a current trend to focus on the measurement and validation of individual isolated applications (e.g., trusted execution) due to the high complexity of verifying the complete software state of a device. Furthermore, the extension of remote attestation with runtime program measurements is currently unsolved but is now addressed in embodiments consistent with the present disclosure.

FIG. 2 illustrates example configurations for verifier 104′ and prover 102′ usable in accordance with at least one embodiment of the present disclosure. In particular, verifier 104′ and prover 102′ may be capable of performing example functionality such as disclosed in FIG. 1. However, verifier 104′ and prover 102′ are meant only as examples of apparatuses that may be usable in embodiments consistent with the present disclosure, and are not meant to limit these various embodiments to any particular manner of implementation.

Verifier 104′ may comprise, for example, system module 200 configured to manage device operations. System module 200 may include, for example, processing module 202, memory module 204, power module 206, user interface module 208 and communication interface module 210. Verifier 104′ may also include communication module 212 and TEE module 214. While communication module 212 and TEE module 214 have been illustrated as separate from system module 200, the example implementation shown in FIG. 2 has been provided merely for the sake of explanation. Some or all of the functionality associated with communication module 212 and TEE module 214 may be incorporated into system module 200.

In verifier 104′, processing module 202 may comprise one or more processors situated in separate components, or alternatively, one or more processing cores embodied in a single component (e.g., in a System-on-a-Chip (SoC) configuration) and any processor-related support circuitry (e.g., bridging interfaces, etc.). Example processors may include, but are not limited to, various x86-based microprocessors available from the Intel Corporation including those in the Pentium, Xeon, Itanium, Celeron, Atom, Core i-series product families, Advanced RISC (e.g., Reduced Instruction Set Computing) Machine or “ARM” processors, etc. Examples of support circuitry may include chipsets (e.g., Northbridge, Southbridge, etc. available from the Intel Corporation) configured to provide an interface through which processing module 202 may interact with other system components that may be operating at different speeds, on different buses, etc. in verifier 104′. Some or all of the functionality commonly associated with the support circuitry may also be included in the same physical package as the processor (e.g., such as in the Sandy Bridge family of processors available from the Intel Corporation).

Processing module 202 may be configured to execute various instructions in verifier 104′. Instructions may include program code configured to cause processing module 202 to perform activities related to reading data, writing data, processing data, formulating data, converting data, transforming data, etc. Information (e.g., instructions, data, etc.) may be stored in memory module 204. Memory module 204 may comprise random access memory (RAM) or read-only memory (ROM) in a fixed or removable format. RAM may include volatile memory configured to hold information during the operation of verifier 104′ such as, for example, static RAM (SRAM) or Dynamic RAM (DRAM). ROM may include non-volatile (NV) memory modules configured based on BIOS, UEFI, etc. to provide instructions when verifier 104′ is activated, programmable memories such as electronic programmable ROMs (EPROMS), Flash, etc. Other fixed/removable memory may include, but are not limited to, magnetic memories such as, for example, floppy disks, hard drives, etc., electronic memories such as solid state flash memory (e.g., embedded multimedia card (eMMC), etc.), removable memory cards or sticks (e.g., micro storage device (uSD), USB, etc.), optical memories such as compact disc-based ROM (CD-ROM), Digital Video Disks (DVD), Blu-Ray Disks, etc.

Power module 206 may include internal power sources (e.g., a battery, fuel cell, etc.) and/or external power sources (e.g., electromechanical or solar generator, power grid, fuel cell, etc.), and related circuitry configured to supply verifier 104′ with the power needed to operate. User interface module 208 may include hardware and/or software to allow users to interact with verifier 104′ such as, for example, various input mechanisms (e.g., microphones, switches, buttons, knobs, keyboards, speakers, touch-sensitive surfaces, one or more sensors configured to capture images and/or sense proximity, distance, motion, gestures, orientation, etc.) and various output mechanisms (e.g., speakers, displays, lighted/flashing indicators, electromechanical components for vibration, motion, etc.). The hardware in user interface module 208 may be incorporated within verifier 104′ and/or may be coupled to verifier 104′ via a wired or wireless communication medium.

Communication interface module 210 may be configured to manage packet routing and other control functions for communication module 212, which may include resources configured to support wired and/or wireless communications. In some instances, verifier 104′ may comprise more than one communication module 212 (e.g., including separate physical interface modules for wired protocols and/or wireless radios) all managed by a centralized communication interface module 210. Wired communications may include serial and parallel wired mediums such as, for example, Ethernet, Universal Serial Bus (USB), Firewire, Digital Video Interface (DVI), High-Definition Multimedia Interface (HDMI), etc. Wireless communications may include, for example, close-proximity wireless mediums (e.g., radio frequency (RF) such as based on the Near Field Communications (NFC) standard, infrared (IR), etc.), short-range wireless mediums (e.g., Bluetooth, WLAN, Wi-Fi, etc.), long range wireless mediums (e.g., cellular wide-area radio communication technology, satellite-based communications, etc.) or electronic communications via sound waves. In one embodiment, communication interface module 210 may be configured to prevent wireless communications that are active in communication module 212 from interfering with each other. In performing this function, communication interface module 210 may schedule activities for communication module 212 based on, for example, the relative priority of messages awaiting transmission. While the embodiment disclosed in FIG. 2 illustrates communication interface module 210 being separate from communication module 212, it may also be possible for the functionality of communication interface module 210 and communication module 212 to be incorporated into the same module.

TEE module 214 may be a secure workspace in which known-good programs may execute, confidential information may be stored in a secure manner, etc. In general, TEE module 214 may comprise a set of computing resources that are secure such that programs executing within TEE module 214, and any data associated with the executing programs, are isolated. The programs/data cannot be interfered with or observed during program execution with the exception that the program may be started or stopped and the associated data may be inserted or deleted. The insertion of data may be unobserved, and thus not interfered with, and any data leaving TEE module 214 is released in a controlled manner. Consistent with the present disclosure, at least one known-good program executing within TEE module 214 may perform any or all operations disclosed herein in regard to TEE module 106. In one example implementation, TEE module 214 may utilize Software Guard Extensions (SGX) technology developed by the Intel Corporation. SGX may provide a secure and hardware-encrypted computation and storage area inside of the system memory, the contents of which cannot be deciphered by privileged code or even through the application of hardware probes to memory bus. When TEE module 214 is protected by SGX, embodiments consistent with the present disclosure make it impossible for an intruder to decipher the contents of TEE module 214. Protected data cannot be observed outside of SGX, and thus, is inaccessible outside of SGX.

In an example implementation wherein TEE module 214 is implemented using SGX, the identity of programs (e.g., based on a cryptographic hash measurement of each program's contents) may be signed and stored inside each program. When the programs are then loaded, the processor verifies that the measurement of the program (e.g., as computed by the processor) is identical to the measurement previously embedded inside the program. The signature used to sign the embedded measurement is also verifiable because the processor is provided with a public key used to verify the signature at program load time. This way malware can't tamper with the program without also altering its verifiable measurement. Malware also cannot spoof the signature because the signing key is secure with the program's author. Thus, the software may not be read, written to or altered by any malware. Moreover, data may also be protected in TEE module 106. For example, known-good programs in TEE module 106 may encrypt data such as keys, passwords, licenses, etc. so that only verified good programs may decrypt this data.

Attestation module 216 may reside within TEE module 214. Attestation module 216 may interact with, for example, processing module 202, memory module 204 and communication module 212. In an example of operation, attestation module 216 may cause communication module 212 to send challenges to prover 102′ and may receive attestation and ticket data from prover 102′. Attestation module 216 may utilize processing resources in processing module 202 and/or memory resources in memory module 204 to process the data received from prover 102′. Moreover, attestation module 216 may cause communication module 212 to transmit at least the ticket data to verifier 104 and may receive authorization token information from verifier 104 for use in generating symmetric keys 114. Attestation module 216 may then cause communication module 212 to transmit at least one symmetric key 114 to prover 102′.

Prover 102′ has been represented as a resource-limited device in the example of FIG. 2. Communication module 218 may be configured to transmit and receive data to at least verifier 104′ (e.g., communication module 212). Data received by communication module 218 may be provided to operational resources in prover 102′ (e.g., to consolidated operations module 220). Consolidated operations module 220 may be able to perform functionality similar to system module 200 but on a smaller scale. For example, consolidated operations module 220 may comprise an all-in-one processing solution such as an SoC that may be configured to handle all of the processing operations for prover 102′ including resources such as, for example, processor 222, platform initialization module (e.g., a boot loader) 224, memory 226, etc. In one embodiment, consolidated operations module 220 may cause communication module 218 to interact with communication module 212. In one embodiment, this interaction may be limited to wired communication, close-proximity or short-range wireless communication. While this limitation may simply be a functional reality of prover 102′, it may also serve to enhance the security of the interaction between verifier 104′ and prover 102′ in that prover 102′ may only be able to interact with a single device in close range (e.g., verifier 104′) during symmetric key distribution 108.

FIG. 3 illustrates an example of symmetric key distribution 108′ in accordance with at least one embodiment of the present disclosure. FIG. 3 details how verifier 104 and prover 102 may establish a shared symmetric key K_(V,P) that may be used for remote attestation, an example of which will be described in FIG. 3. Certifier 106 (e.g., the manufacturer of prover 102) may determine an identification ID_(P) of prover 102 (e.g., at manufacturing time), and may embed an authentication key EK_(P) into a secure memory location (e.g., encrypted memory) in prover 102. To enhance security, it may be assumed that prover 102 cannot communicate with another device besides verifier 104 during the course of the protocol exchange (e.g., via wired, close-proximity or short-range communication), thus preventing a possible collusion of compromised software at prover 102 with certifier 106 or another external entity. However, verifier 104 and certifier 106 may interact via a secure channel, such as Transport Layer Security (TLS) over Transport Control Protocol/Internet Protocol (TCP/IP). As shown in FIG. 3, verifier 104 may generate random numbers N_(V) and N_(AUTH) for the key distribution session, and may hash these values with an identifier for verifier 104 ID_(y) for transmitting the device challenge. Prover 102 may generate data for responding to the challenge, the response including at least key data C_(P) and ticket_(V,M). C_(P) and ticket_(V,M) may be generated in prover 102 by performing cryptographic functions on various values including at least the values sent in the device challenge and ID_(P).

In one embodiment, verifier 104 may then forward N_(AUTH) and ticket_(V,M) to certifier 106 (e.g., via a secure channel). A master-slave key scheme may be employed to reduce the required storage so that certifier 106 may be able to handle a large number of provers 102. Knowing E_(K), certifier 106 may be able to derive EK_(P) for one of its devices ID_(P) using a key derivation function (KDF) such as EK_(P)=KDF(E_(K), ID_(P)). Authentication of ticket_(V,M) by certifier 106 may result in the generation of authorization token S_(V) wherein S_(V)=ADEC(E_(KP), ticket_(V,M)). At least S_(V) may then be transmitted back to verifier 104 for use in setting symmetric key K_(V,P). As shown in FIG. 3, confirmation CV may then be generated based on C_(V)=MAC (K_(V,P), N_(V), N_(P)). Prover 102 may receive confirmation C_(V) and may utilize C_(V) to determine that symmetric key K_(V,P) has been set.

In the example protocol of FIG. 3, verifier 104 may not be explicitly authenticated towards prover 102 or certifier 106, meaning that any verifier 104 may trigger this protocol. However, this exchange assures that certifier 106 learns the identity ID_(y) associated with the symmetric key K_(V,P) deployed to ID_(P). As such, at least two alternatives may be available for out-of-band authentication (and authorization) of verifier 104: a distributed method that may exploit local (e.g., physical) authorization between verifier 104 and prover 102, and a centralized method wherein certifier 106 may act as a mediator between verifier 104 and prover 102. Local authorization may leverage that many devices already provide an out-of-band key deployment based on physical proximity or similar relationships. For example, a device pairing protocol may perform a symmetric key exchange in a local trusted environment, and may then ask the user to authenticate the exchange by entering a device PIN on either device. In this manner, a common shared secret “S” can be established with prover 102 that was newly acquired by verifier 104. However, prover 102 needs to be equipped with trusted user I/O facilities to perform the pairing in the trusted environment (e.g., which may not be available in some limited-resource devices). Otherwise, verifier 104 would not know if S is shared with secure or compromised software on prover 102. While S by itself may not be useful for attestation, S can be used in the example of FIG. 3 to establish an authenticated channel between verifier 104 and prover 102, ensuring that prover 102 cannot communicate with other parties except for verifier 104. In this manner, S may be used to authenticate and authorize the deployment of symmetric key K_(V,P) locally between verifier 104 and prover 102. The physical isolation of prover in the base protocol ensures that no collaboration with certifier 106 can occur and a successfully shared symmetric key K_(V,P) is only known to verifier 104 and prover 102. Hence, even if prover 102 was compromised, symmetric key K_(V,P) can be used to attest prover 102 and to detect the compromise. Physical isolation may no longer be necessary after attestation of prover 102.

If a local authorization is not feasible or not deemed sufficiently secure, the centralized solution may be employed where certifier 106 explicitly authorizes a verifier 104 to establish a symmetric key K_(V,P) with prover 102. For this purpose, verifier should authenticate to certifier 106 when establishing the secure channel in the base protocol, such that the identity of verifier 104 may be cryptographically linked to ID_(V) as used in the local communication between verifier 104 and prover 102. For example, if the secure channel between verifier 104 and certifier 106 uses a standard public key scheme for mutual asymmetric authentication, ID_(V) may be the fingerprint (e.g., cryptographic hash) of the public key V_(pub) of verifier 104. Certifier 106 may then verify that prover 102 is interacting with the same verifier 104 that is in communication with certifier 106 based on no other entity besides verifier 104 knowing the private key for V_(pub), which may be uniquely identified by ID_(V)=hash(V_(pub)). Hence, certifier 106 may perform an authorization of verifier 104 based on ID_(V) and ID_(P), (e.g., by looking up in a database if verifier 104 has purchased prover 102 or if verifier 104 is part of a certain privileged group that can perform attestation of P).

FIG. 4 illustrates an example of remote attestation 112′ in accordance with at least one embodiment of the present disclosure. The remote attestation protocol is a challenge-response protocol where prover 102 may “certify” the software state of its platform using the deployed symmetric key K_(V,P). For this purpose, traditional attestation schemes record the software state of prover 102 in a secure memory location on prover 102, such as the platform configuration registers (PCRs) of a TPM, and then sign those recorded measurements together with a nonce from a challenge by verifier 104. The complete record from an initial bootstrap routine to launching of the OS and applications is called a chain of trust, as each component requires the previously loaded component to measure it.

A similar scheme can be realized by storing software measurements in a memory location that is protected with Execution-aware memory protection scheme by protecting the attestation key K_(V,P) as well as the “software PCRs” such that only the trusted remote attestation code can access that data. However, embedded environments and symmetric attestation keys also enable a further simplification, which may be considered an “implicit” or “derived” chain of trust. In this approach, each time some software is loading another software module the chain of trust may be extended using a corresponding key generation and/or key delegation operation instead of storing the measurements in a safe location. As show in FIG. 4, in prover 102″ the chain of trust may be started by the platform bootstrapping code which is in possession of the root symmetric key K_(V,P). However, instead of providing a runtime service based on secure hardware or firmware, which may be resource intensive, a secure boot routine may measure (e.g., typically by hashing) the software module and directly embeds a derived program key K_(M)=MAC(K_(V,P), hash_(M)) into a reserved location of the loaded program module. Examples operations are illustrated in FIG. 4 wherein for boot loader 400, Hash₁=measurement (boot loader code) and program key K₁=MAC(K_(V,P), Counter∥Hash₁). Program key K₁ may then be employed to generate a program key corresponding to OS/Task 402 (e.g., Hash₂=measurement (OS/Task code) and program key K₂=MAC(K₁, Counter∥Hash₂). Likewise, a program key corresponding specifically to other software 404 may also be derived, wherein Hash₃=measurement (other software code) and program key K₃=MAC(K₁, Counter∥Hash₃).

Each software module (e.g., OS/Task 402, other software 404, etc.) may then attest to its initial state (e.g., measurement) directly using their corresponding program key K_(M) (e.g., K₁, K₂, K₃, etc.) together with a chain of measurements (e.g., the various hashes stored in each module). Receiving these measurements as part of the response provided by prover 102 to the application challenge, verifier 104 may then be able to derive program key K_(M) from K_(V,P) and verify the correct derivation of K_(M) based only on a MAC computed with program key K_(M) in view of the nonce provided by verifier 104 in the application challenge. This approach may be especially useful in, for example, low-end devices enabled by execution-aware memory protection that do not require a trusted OS, trusted firmware or privileged CPU modes to provide trusted execution for individual embedded applications (e.g., at minimum hardware and software cost). Symmetric chain of trust may also be used to generate a software-specific encryption key (e.g., used for “sealing” the software).

FIG. 5 illustrates example operations for symmetric keying and chain of trust from the perspective of a verifier in accordance with at least one embodiment of the present disclosure. Initially, attestation may be triggered in verifier. Example attestation triggers may include, for example, the activation of the verifier or a prover, a request to access resources in a verifier received from the prover, data being received from the prover, etc. A determination may then be made in operation 502 as to whether key distribution is required. Key distribution may be required if, for example, the verifier has never interacted with the prover, existing symmetric keys are corrupt or expired, etc. If it is determined in operation 502 that key distribution is required, then in operation 504 a device challenge may be transmitted to the prover. A response to the device challenge may be received in operation 506, the response comprising at least key data and ticket data. The verifier may then proceed to transmit at least the ticket data to a certifier in operation 508. A determination may then be made in operation 510 as to whether a message containing at least an authorization token was received from the certifier in response to the transmission of at least the ticket information in operation 508. If in operation 510 it is determined that a message including the authorization token was not received, then in operation 512 key distribution operations may terminate or optionally return to operation 504 for another attempt. If in operation 510 it is determined that a message comprising at least the authorization token was received, then in operation 514 the authorization token may be used to set a symmetric key (e.g., based at least on the key data received in operation 506). A confirmation that the key was set may then be transmitted to the prover in operation 516.

If in operation 502 it is determined that attestation is required for application and/or data authentication (e.g., that key distribution is not required), then in operation 518 an application challenge may be transmitted to the prover. A response to the application challenge may then be received in operation 520. The verifier may proceed to authenticate the contents of the response in operation 522. Authenticating the contents of the response may comprise at least performing a MAC on a program key provided in the response to determine if the program key was generated based on the symmetric key that was set in operation 514. A reply based on the authentication may be determined in operation 524 and then transmitted to the prover in operation 526.

FIG. 6 illustrates example operations for symmetric keying and chain of trust from the perspective of a prover in accordance with at least one embodiment of the present disclosure. The operations disclosed in FIG. 6 presume that key distribution operations (e.g., such as disclosed in FIG. 5, operations 504 to 518) have already occurred so that a symmetric key already resides in the prover. The prover may be activated in operation 600. Following prover activation, bootstrap loading may occur in operation 602 wherein, for example, a framework key (e.g., a boot loader key) may be generated based at least on a counter, a measurement of the boot loader code and the symmetric key.

After the counter is incremented in operation 604, a next code module may be loaded in operation 606. For example, an OS/task module may be loaded following the boot code, which may then be followed by other software being loaded, etc. in the prover. Following the loading of the next code module in operation 606, a program key for the code module may be generated based on a current value of the counter, a measurement of the current code module being loaded and the key generated for the previous code module (e.g., framework key or a previous program key) in operation 608. A determination may then be made in operation 610 as to whether there are additional code modules to load in the prover. A determination in operation 610 that there are still additional code modules to load may be followed by a return to operation 604 wherein the counter may be incremented for the next code module. Otherwise, if it is determined in operation 610 that all of the code modules have been loaded, then in operation 612 the prover may start to listen for challenges from the verifier.

While FIGS. 5 and 6 illustrate operations according to different embodiments, it is to be understood that not all of the operations depicted in FIGS. 5 and 6 are necessary for other embodiments. Indeed, it is fully contemplated herein that in other embodiments of the present disclosure, the operations depicted in FIGS. 5 and 6, and/or other operations described herein, may be combined in a manner not specifically shown in any of the drawings, but still fully consistent with the present disclosure. Thus, claims directed to features and/or operations that are not exactly shown in one drawing are deemed within the scope and content of the present disclosure.

As used in this application and in the claims, a list of items joined by the term “and/or” can mean any combination of the listed items. For example, the phrase “A, B and/or C” can mean A; B; C; A and B; A and C; B and C; or A, B and C. As used in this application and in the claims, a list of items joined by the term “at least one of” can mean any combination of the listed terms. For example, the phrases “at least one of A, B or C” can mean A; B; C; A and B; A and C; B and C; or A, B and C.

As used in any embodiment herein, the term “module” may refer to software, firmware and/or circuitry configured to perform any of the aforementioned operations. Software may be embodied as a software package, code, instructions, instruction sets and/or data recorded on non-transitory computer readable storage mediums. Firmware may be embodied as code, instructions or instruction sets and/or data that are hard-coded (e.g., nonvolatile) in memory devices. “Circuitry”, as used in any embodiment herein, may comprise, for example, singly or in any combination, hardwired circuitry, programmable circuitry such as computer processors comprising one or more individual instruction processing cores, state machine circuitry, and/or firmware that stores instructions executed by programmable circuitry. The modules may, collectively or individually, be embodied as circuitry that forms part of a larger system, for example, an integrated circuit (IC), system on-chip (SoC), desktop computers, laptop computers, tablet computers, servers, smartphones, etc.

Any of the operations described herein may be implemented in a system that includes one or more storage mediums (e.g., non-transitory storage mediums) having stored thereon, individually or in combination, instructions that when executed by one or more processors perform the methods. Here, the processor may include, for example, a server CPU, a mobile device CPU, and/or other programmable circuitry. Also, it is intended that operations described herein may be distributed across a plurality of physical devices, such as processing structures at more than one different physical location. The storage medium may include any type of tangible medium, for example, any type of disk including hard disks, floppy disks, optical disks, compact disk read-only memories (CD-ROMs), compact disk rewritables (CD-RWs), and magneto-optical disks, semiconductor devices such as read-only memories (ROMs), random access memories (RAMs) such as dynamic and static RAMs, erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), flash memories, Solid State Disks (SSDs), embedded multimedia cards (eMMCs), secure digital input/output (SDIO) cards, magnetic or optical cards, or any type of media suitable for storing electronic instructions. Other embodiments may be implemented as software modules executed by a programmable control device.

Thus, the present application is directed to symmetric keying and chain of trust. In one embodiment a prover may communicate with a verifier to perform device or data authentication. The verifier may issue a challenge to authenticate the prover, to which the prover may respond with key data and ticket data that may be forwarded to a certifier. If the ticket data is recognized, the certifier may transmit an authorization token to the verifier, which may then set a symmetric attestation key. The prover may utilize the symmetric attestation key to establish a chain of trust for use in responding to challenges to application or data authenticity. The symmetric attestation key may be used to generate a first level key that may be utilized to generate program keys. A challenge response may include at least program key data that may be authenticated by the verifier using the symmetric attestation key.

The following examples pertain to further embodiments. The following examples of the present disclosure may comprise subject material such as a device, a method, at least one machine-readable medium for storing instructions that when executed cause a machine to perform acts based on the method, means for performing acts based on the method and/or a system for symmetric keying and chain of trust, as provided below.

According to example 1 there is provided a device to act as a verifier. The device may comprise a communication module and a trusted execution environment including at least an attestation module to detect an attestation trigger, determine whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger and initiate at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.

Example 2 may include the elements of example 1, wherein attestation is triggered periodically or due to a request to access resources received from the prover device via the communication module.

Example 3 may include the elements of any of examples 1 to 2, wherein the communication module is to interact with the prover device via at least one of a short-range wireless connection or a direct wired connection.

Example 4 may include the elements of any of examples 1 to 2, wherein the attestation module being to initiate the symmetric key distribution operations with the prover device comprises the attestation module being to cause the communication module to transmit a device challenge to the prover device and receive a response to the device challenge from the prover device via the communication module, the response comprising at least key data and ticket data.

Example 5 may include the elements of example 4, wherein the attestation module is further to cause the communication module to transmit at least the ticket data to at least one certifier device, receive a response from the at least one certifier device via the communication module and determine whether the response received from the certifier device comprises an authorization token.

Example 6 may include the elements of example 5, and may further comprise a firewall through which the communication module interacts with the at least one certifier device, the firewall being to at least prevent unintended interaction between the device and the at least one certifier device.

Example 7 may include the elements of example 5, wherein the attestation module is further to, if an authorization token is determined to have been received, set a symmetric key based at least on the key data and cause the communication module to transmit a confirmation that the symmetric key has been set to the prover device.

Example 8 may include the elements of example 7, wherein the attestation module being to initiate the remote attestation operations with the prover device comprises the attestation module being to cause the communication module to transmit an application challenge to the prover device, receive a response to the application challenge from the prover device via the communication module, the response being generated based at least on counter data, code measurement data and program key data, authenticate the response and cause the communication module to transmit at least the results of the authentication to the prover device.

Example 9 may include the elements of example 8, wherein the attestation module being to authenticate the response comprises the attestation module being to authenticate that the response was generated based on the symmetric key using a message authentication code.

According to example 10 there is provided method for remote attestation using symmetric keys. The method may comprise detecting an attestation trigger, determining whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger and initiating at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.

Example 11 may include the elements of example 10, wherein attestation is triggered periodically or due to a request to access resources received from the prover device.

Example 12 may include the elements of any of examples 10 to 11, and may further comprise interacting with the prover device via at least one of a short-range wireless connection or a direct wired connection.

Example 13 may include the elements of any of examples 10 to 11, wherein initiating symmetric key distribution operations with the prover device comprises causing a device challenge to be transmitted to the prover device and receiving a response to the device challenge from the prover device, the response comprising at least key data and ticket data.

Example 14 may include the elements of example 13, and may further comprise causing at least the ticket data to be transmitted to at least one certifier device, receiving a response from the at least one certifier device and determining whether the response received from the at least one certifier device comprises an authorization token.

Example 15 may include the elements of example 14, and may further comprise, if an authorization token is determined to have been received, setting a symmetric key based at least on the key data and causing a confirmation that the symmetric key has been set to be transmitted to the prover device.

Example 16 may include the elements of example 15, wherein initiating remote attestation operations with the prover comprises causing an application challenge to be transmitted to the prover device, receiving a response to the application challenge from the prover device, the response being generated based at least on counter data, code measurement data and program key data, authenticating the response and causing at least the results of the authentication to be transmitted to the prover device.

Example 17 may include the elements of example 16, wherein authenticating the response comprises authenticating that the response was generated based on the symmetric key using a message authentication code.

According to example 18 there is provided a system including at least a device, the system being arranged to perform the method of any of the above examples 10 to 17.

According to example 19 there is provided a chipset arranged to perform the method of any of the above examples 10 to 17.

According to example 20 there is provided at least one machine readable medium comprising a plurality of instructions that, in response to be being executed on a computing device, cause the computing device to carry out the method according to any of the above examples 10 to 17.

According to example 21 there is provided a device configured for remote attestation using symmetric keys, the device being arranged to perform the method of any of the above examples 10 to 17.

According to example 22 there is provided a system for remote attestation using symmetric keys, comprising means for detecting an attestation trigger, means for determining whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger and means for initiating at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.

Example 23 may include the elements of example 22, wherein attestation is triggered periodically or due to a request to access resources received from the prover device.

Example 24 may include the elements of any of examples 22 to 23, and may further comprise means for interacting with the prover device via at least one of a short-range wireless connection or a direct wired connection.

Example 25 may include the elements of any of examples 22 to 23, wherein the means for initiating symmetric key distribution operations with the prover device comprise means for causing a device challenge to be transmitted to the prover device and means for receiving a response to the device challenge from the prover device, the response comprising at least key data and ticket data.

Example 26 may include the elements of example 25, and may further comprise means for causing at least the ticket data to be transmitted to at least one certifier device, means for receiving a response from the at least one certifier device and means for determining whether the response received from the at least one certifier device comprises an authorization token.

Example 27 may include the elements of example 26, and may further comprise means for, if an authorization token is determined to have been received, setting a symmetric key based at least on the key data and means for causing a confirmation that the symmetric key has been set to be transmitted to the prover device.

Example 28 may include the elements of example 27, wherein the means for initiating remote attestation operations with the prover comprise means for causing an application challenge to be transmitted to the prover device, means for receiving a response to the application challenge from the prover device, the response being generated based at least on counter data, code measurement data and program key data, means for authenticating the response and means for causing at least the results of the authentication to be transmitted to the prover device.

Example 29 may include the elements of example 28, wherein the means for authenticating the response comprise means for authenticating that the response was generated based on the symmetric key using a message authentication code.

According to example 30 there is provided a device to act as a prover. The device may comprise a communication module and at least operational resources to establish a symmetric chain of trust, the operational resources including at least a platform initialization module to store a symmetric key generated as a result of symmetric key distribution operations performed in conjunction with a verifier device via the communications module.

Example 31 may include the elements of example 30, wherein the platform initialization module is further to measure a code module to be loaded on the device and provision a program key to a runtime representation of the code module loaded on the device based at least on the symmetric key and the code module measurement.

Example 32 may include the elements of example 31, wherein the platform initialization module being to provision the program key to the runtime representation of the code module comprises the platform initialization module being to utilize a message authentication code to generate the program key based on the symmetric key, the code module measurement and a counter value.

Example 33 may include the elements of example 32 wherein the platform initialization module is further to receive an application challenge from the verifier device via the communication module, the application challenge being directed to the runtime representation of the code module and transmit a response to the application challenge via the communication module, the response being based on at least the program key, the code module measurement and the counter value.

Example 34 may include the elements of any of examples 32 to 33, wherein the platform initialization module is further to increment the value of the counter, measure a second code module to be loaded on the device and provision a second program key to a runtime representation of the second code module loaded on the device based at least on the program key, the second code module measurement and the counter value.

Example 35 may include the elements of example 34, wherein the platform initialization module being to provision the second program key to the runtime representation of the second program comprises the platform initialization module being to utilize a message authentication code to generate the second program key based on the program key, the second code module measurement and the counter value.

Example 36 may include the elements of example 35 wherein the platform initialization module is further to receive an application challenge from the verifier device via the communication module, the application challenge being directed to the runtime representation of the second code module, and transmit a response to the application challenge via the communication module, the response being based on at least the second program key, the second code module measurement and the counter value.

According to example 37 there is provided a method for establishing a chain of trust. The method may comprise storing a symmetric key in a prover device, the symmetric being generated as a result of symmetric key distribution operations performed in conjunction with a verifier device.

Example 38 may include the elements of example 37, and may further comprise measuring a code module to be loaded on the prover device and provisioning a program key to a runtime representation of the code module loaded on the prover device based at least on the symmetric key and the program measurement.

Example 39 may include the elements of example 38, wherein provisioning the program key to the runtime representation of the code module comprises utilizing a message authentication code to generate the program key based on the symmetric key, the code module measurement and a counter value.

Example 40 may include the elements of example 39, further comprising receiving an application challenge from the verifier device, the application challenge being directed to the runtime representation of the code module and transmitting a response to the application challenge, the response being based on at least the program key, the code module measurement and the counter value.

Example 41 may include the elements of any of examples 39 to 40, and may further comprise incrementing the value of the counter, measuring a second code module to be loaded on the prover device and provisioning a second program key to a runtime representation of the second code module loaded on the prover device based at least on the program key, the second code module measurement and the counter value.

Example 42 may include the elements of example 41, wherein provisioning the second program key to the runtime representation of the second program comprises utilizing a message authentication code to generate the second program key based on the program key, the second code module measurement and the counter.

Example 43 may include the elements of example 42, and may further comprise receiving an application challenge from the verifier device, the application challenge being directed to the runtime representation of the second code module and transmitting a response to the application challenge, the response being based on at least the second program key, the second code module measurement and the counter value.

According to example 44 there is provided a system including at least a device, the system being arranged to perform the method of any of the above examples 37 to 43.

According to example 45 there is provided a chipset arranged to perform the method of any of the above examples 37 to 43.

According to example 46 there is provided at least one machine readable medium comprising a plurality of instructions that, in response to be being executed on a computing device, cause the computing device to carry out the method according to any of the above examples 37 to 43.

According to example 47 there is provided a device configured for remote attestation using symmetric keys, the device being arranged to perform the method of any of the above examples 37 to 43.

According to example 48 there is provided a system for establishing a chain of trust. The system may comprise means for storing a symmetric key in a prover device, the symmetric being generated as a result of symmetric key distribution operations performed in conjunction with a verifier device.

Example 49 may include the elements of example 48, and may further comprise means for measuring a code module to be loaded on the prover device and means for provisioning a program key to a runtime representation of the code module loaded on the prover device based at least on the symmetric key and the program measurement.

Example 50 may include the elements of example 49, wherein the means for provisioning the program key to the runtime representation of the code module comprise means for utilizing a message authentication code to generate the program key based on the symmetric key, the code module measurement and a counter value.

Example 51 may include the elements of example 50, and may further comprise means for receiving an application challenge from the verifier device, the application challenge being directed to the runtime representation of the code module and means for transmitting a response to the application challenge, the response being based on at least the program key, the code module measurement and the counter value.

Example 52 may include the elements of any of examples 50 to 51, and may further comprise means for incrementing the value of the counter; means for measuring a second code module to be loaded on the prover device and means for provisioning a second program key to a runtime representation of the second code module loaded on the prover device based at least on the program key, the second code module measurement and the counter value.

Example 53 may include the elements of example 52, wherein the means for provisioning the second program key to the runtime representation of the second program comprise means for utilizing a message authentication code to generate the second program key based on the program key, the second code module measurement and the counter.

Example 54 may include the elements of example 53, and may further comprise means for receiving an application challenge from the verifier device, the application challenge being directed to the runtime representation of the second code module and means for transmitting a response to the application challenge, the response being based on at least the second program key, the second code module measurement and the counter value.

The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described (or portions thereof), and it is recognized that various modifications are possible within the scope of the claims. Accordingly, the claims are intended to cover all such equivalents. 

What is claimed:
 1. A device to act as a verifier, comprising: a communication module; and a trusted execution environment including at least an attestation module to: detect an attestation trigger; determine whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger; and initiate at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.
 2. The device of claim 1, wherein attestation is triggered periodically or due to a request to access resources received from the prover device via the communication module.
 3. The device of claim 1, wherein the communication module is to interact with the prover device via at least one of a short-range wireless connection or a direct wired connection.
 4. The device of claim 1, wherein the attestation module being to initiate the symmetric key distribution operations with the prover device comprises the attestation module being to: cause the communication module to transmit a device challenge to the prover device; and receive a response to the device challenge from the prover device via the communication module, the response comprising at least key data and ticket data.
 5. The device of claim 4, wherein the attestation module is further to: cause the communication module to transmit at least the ticket data to at least one certifier device; receive a response from the at least one certifier device via the communication module; and determine whether the response received from the certifier device comprises an authorization token.
 6. The device of claim 5, further comprising a firewall through which the communication module interacts with the at least one certifier device, the firewall being to at least prevent unintended interaction between the device and the at least one certifier device.
 7. The device of claim 5, wherein the attestation module is further to: if an authorization token is determined to have been received, set a symmetric key based at least on the key data; and cause the communication module to transmit a confirmation that the symmetric key has been set to the prover device.
 8. The device of claim 7, wherein the attestation module being to initiate the remote attestation operations with the prover device comprises the attestation module being to: cause the communication module to transmit an application challenge to the prover device; receive a response to the application challenge from the prover device via the communication module, the response being generated based at least on counter data, code measurement data and program key data; authenticate the response; and cause the communication module to transmit at least the results of the authentication to the prover device.
 9. The device of claim 8, wherein the attestation module being to authenticate the response comprises the attestation module being to authenticate that the response was generated based on the symmetric key using a message authentication code.
 10. A method for remote attestation using symmetric keys, comprising: detecting an attestation trigger; determining whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger; and initiating at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.
 11. The method of claim 10, wherein attestation is triggered periodically or due to a request to access resources received from the prover device.
 12. The method of claim 10, further comprising: interacting with the prover device via at least one of a short-range wireless connection or a direct wired connection.
 13. The method of claim 10, wherein initiating symmetric key distribution operations with the prover device comprises: causing a device challenge to be transmitted to the prover device; and receiving a response to the device challenge from the prover device, the response comprising at least key data and ticket data.
 14. The method of claim 13, further comprising: causing at least the ticket data to be transmitted to at least one certifier device; receiving a response from the at least one certifier device; and determining whether the response received from the at least one certifier device comprises an authorization token.
 15. The method of claim 14, further comprising: if an authorization token is determined to have been received, setting a symmetric key based at least on the key data; and causing a confirmation that the symmetric key has been set to be transmitted to the prover device.
 16. The method of claim 15, wherein initiating remote attestation operations with the prover comprises: causing an application challenge to be transmitted to the prover device; receiving a response to the application challenge from the prover device, the response being generated based at least on counter data, code measurement data and program key data; authenticating the response; and causing at least the results of the authentication to be transmitted to the prover device.
 17. The method of claim 16, wherein authenticating the response comprises authenticating that the response was generated based on the symmetric key using a message authentication code.
 18. At least one machine-readable storage medium having stored thereon, individually or in combination, instructions that when executed by one or more processors result in the following operations for remote attestation using symmetric keys, comprising: detecting an attestation trigger; determining whether to initiate symmetric key distribution operations or remote attestation operations with a prover device based at least on the attestation trigger; and initiating at least one of symmetric key distribution operations or remote attestation operations with the prover device based on the determination.
 19. The medium of claim 18, wherein attestation is triggered periodically or due to a request to access resources received from the prover device.
 20. The medium of claim 18, further comprising instructions that when executed by one or more processors result in the following operations, comprising: interacting with the prover device via at least one of a short-range wireless connection or a direct wired connection.
 21. The medium of claim 18, wherein initiating symmetric key distribution operations with the prover device comprises: causing a device challenge to be transmitted to the prover device; and receiving a response to the device challenge from the prover device, the response comprising at least key data and ticket data.
 22. The medium of claim 21, further comprising instructions that when executed by one or more processors result in the following operations, comprising: causing at least the ticket data to be transmitted to at least one certifier device; receiving a response from the at least one certifier device; and determining whether the response received from the at least one certifier device comprises an authorization token.
 23. The medium of claim 22, further comprising instructions that when executed by one or more processors result in the following operations, comprising: if an authorization token is determined to have been received, setting a symmetric key based at least on the key data; and causing a confirmation that the symmetric key has been set to be transmitted to the prover device.
 24. The medium of claim 23, wherein initiating remote attestation operations with the prover comprises: causing an application challenge to be transmitted to the prover device; receiving a response to the application challenge from the prover device, the response being generated based at least on counter data, code measurement data and program key data; authenticating the response; and causing at least the results of the authentication to be transmitted to the prover device.
 25. The medium of claim 24, wherein authenticating the response comprises authenticating that the response was generated based on the symmetric key using a message authentication code. 